Lulu.com is again the best choice I've found because of the low investment and quick turnaround.
If you're going to get your book listed in the major bookstores then you need an ISBN. If you go through the "normal" channels you'll be faced with a $300+ price tag to get the ISBN.
Here's the Official ISBN site http://www.isbn.org/ however, don't buy from there.
On this site you will find a form that will cost you an investment of around $300 and at least 10-14 days.
Several years ago the only options you had were to go through a traditional publisher or pay this outrageous price to get your ISBN so you can be listed in online and offline bookstores.
Not any more.
Now, using LuLu.com (http://www.lulu.com/) you can get an ISBN for as low as $34.95 – and have it within 24 hours:
Lulu.com offers the $34.95 option if you just want an ISBN number or for only $149.95 you get...
"Your book is entered into Ingram's database, the largest US book wholesaler. This gets your book into the same wholesale channels as major US publishers and will allow book retailers like Amazon, B&N, and Borders to sell your book. Your title will also be made available in the UK through Bertrams and Gartners, who are wholesalers in the UK market. Your title will be entered into Books In Print, Titles@Ingram, and Neilsons Bookdata (UK) bibliographic databases to facilitate the widest availability around the world."
For about 1/2 the price of an ISBN on ISBN.org you can have your ISBN and Global Distribution of your book set up for you.